In the operation of steam turbines in electric power plants, turbine load is determined by controlled flow of steam through steam admission valves. During startup, steam admission flow may be controlled by positioning control of the main high pressure turbine inlet valves. In fossil power plant turbines, the main high pressure turbine inlet valves comprise one or more throttle valves which are provided with a position control pilot valve. Just before or after synchronization, control of the steam admission flow is transferred from the throttle valves to the governor or control valves which are located downstream from the throttle valves in the high pressure turbine inlet flow configuration. Thereafter, the throttle valves are held wide open and turbine loading is controlled by the governor valves which are typically also provided with a position control capability.
In the case of nuclear power plants, the main high pressure turbine inlet valves are typically one or more stop valves instead of position controlled throttle valves. Accordingly, governor valves located downstream from the stop valves are typically position controlled throughout startup and loading operation to provide speed and load control for nuclear turbines.
Similarly, reheat steam flow between high pressure and intermediate pressure turbines passes through reheat stop valves and interceptor valves which can be position controlled. The reheat stop valves are normally open during turbine operation and they are closed only if a turbine trip condition occurs.
In turbine inlet valve configurations like those just described, steam inlet valve availability for turbine control is an important consideration in power plant management since power system security, plant scheduling and plant operating economy are all affected by this factor. Once the turbine is placed in load operation, changes effected in governor valve positions to meet load demand provide continuing checks of governor valve operability. Similarly, the interceptor valves are subject to periodic operation when supplied with position control to provide continuing checks on their operability.
In the case of turbine inlet valves which are held wide open during turbine load operation, no checks are normally made on valve availability as a result of valve operations directed to turbine speed and load control. For example, in the case of base load steam turbines, the period of uninterrupted load operation and inactive throttle valve or stop valve status can be several months or more. Accordingly, if at some unpredictable point in time it becomes necessary to close a throttle or stop valve to avoid turbine overspeed or for other plant purposes, there is relatively less assurance of throttle or stop valve availability as compared to governor or other valves which are position controlled during load operation.
Historically, tests have accordingly been employed for throttle and stop valves during turbine load operations in electric power plants. Typically, the turbine manufacturer recommends the frequency with which a valve test should be made, and that frequency for example could be once per week.
In performing the throttle or stop valve test, it is desirable that the test be performed without disturbing the power generation process regardless of whether the steam admission configuration is the single ended or double ended steam chest type, the Y-type, the in-line type or other types of commercially supplied steam admission valve configurations. Power should continue to be produced by the turbine driven generator even though the valve test is being performed, and no substantial change should occur in the power generation level during the initiation, performance and termination of the valve test. Accordingly, it is generally desirable that automatic load control be operative during valve tests to hold turbine steam and turbine load substantially constant. However, operator manual control can be used to adjust the turbine load as required during a valve test if an automatic load control is not available during the valve test.
Manufacturer recommendations typically specify maximum and minimum loads for performance of throttle or stop valve tests. The manufacturer specified upper load limit for valve testing is normally based on the load pickup capability of parallel inlet steam flow paths in the turbine undergoing throttle or stop inlet valve testing. Therefore, it would not be unusual that it would necessary to redistribute the load demand placed on the turbine before initiation of an inlet valve test so that the turbine load would be in the allowable range. Any cutback in turbine load demand would be made up by increasing the load demand on another turbine in the system.
There are other conditions or circumstances which should also be satisfied in the testing of turbine inlet valves. Thus, a test for valve availability should be performable whether the turbine is in the single or sequential mode of governor valve operation. In the sequential mode, the turbine stresses and shock produced by partial steam admission or operation can become excessive at lower load levels and it is principally this factor which serves as a basis for the manufacturer specified lower load limit for valve testing. Since nuclear turbines typically have only four arcs of steam admission with greater turbine stress and shock in partial arc operation, it would not be unusual for stop valve tests to be limited to single valve operation in nuclear turbines.
In one typical prior art valve test arrangement, each of two single ended steam chests supply turbine steam through four governor valves. A single throttle valve supplies steam to each chest from the plant steam source. An electrohydraulic controller positions the eight governor valves for load control as the two throttle valves are held wide open.
To test the throttle valve associated with one of the steam chests, it is necessary first to close the governor valves downstream from that throttle valve, then make a test closure of the throttle valve and reopen the throttle valve and finally reopen the associated governor valves. The same procedure is repeated for the second steam chest. Governor valve closure prior to throttle valve closure is necessary to avoid a high steam pressure drop across the test throttle valve when it is closed since throttle valves typically are not reopenable against high steam pressure. On the other hand governor valves typically can be reopened against high steam pressure since they typically operate with balanced valve plug forces.
Governor valve closure in the test procedure is typically effected in feedback analog turbine control systems by application of an electrical test bias signal to the electrohydraulic controllers for the governor valves to be closed. As the governor valves are closed, an impulse pressure control loop can automatically cause the remaining governor valves to open wider and meet load demand as the governor valves involved in the throttle valve test are closed. Alternatively, a manual control input can be used to raise the load demand signal artifically high so that the remaining governor valves more or less provide the load actually desired. Once the throttle valve test closure is completed, the test bias is removed from the associated governor valve controllers and turbine load operation is returned to normal. Thus, throttle valve testing can typically be performed with existing electrohydraulic feedback control systems substantially without disturbing existing load. Further, throttle valve testing can typically be initiated in either the sequential or the single valve mode of governor valve operation. Following a test, governor valves in the test path are reopened, and all governor valves smoothly move under feedback control to the positions required to satisfy load demand in the pretest governor valve mode.
In the prior art analog systems, tests for main inlet valve availability without turbine shutdown have been restricted by lower load limits in the sequential governor valve mode. Further, although main inlet valve testing has been generally smoothly implemented, no known provision has been made for implementing main inlet valve testing while operating the downstream valves with feedforward control nor with position management control nor particularly with feedforward position management control. By position managed control, it is meant to refer to a control capability which allows position changes to be made in parallel valves substantially without causing any change in the total steam flow.
Although the patent application Ser. No. 306,752 cross-referenced herein generally discloses a feedforward type digital electrohydraulic turbine control system in which transfers can be made on line between sequential and single valve modes of governor valve operation, that application similarly provides no direction on the testing of inlet stop or throttle valves without turbine shutdown at lower turbine load operating levels in the sequential mode. Nor is there any disclosure provided in that application on the manner in which inlet valve testing can be implemented in a feedforward type control system. The other cross-referenced application Ser. No. 247,877 and related applications cross-referenced therein disclose among other features a digital system for implementing main inlet valve testing specifically in an end bar lift type of inlet valve configuration.
In the present application, no representation is made that any cited prior patent or other art is the best prior art nor that the interpretation placed on such art herein is the only interpretation that can be placed on that art.